De Vrijer, A., & Bishop, D. (2009). The depletion of a fixed anaerobic energy store does not explain task failure during high-intensity exercise. A paper presented at the 14th Annual Congress of the European College of Sport Science, Oslo, Norway, June 24-27.

"Exercise at intensities above VO2max can only be sustained for several minutes. As the energy required to exercise at such intensities can, by definition, not be generated solely by the aerobic system, the cause of task-failure has often been attributed to the exhaustion of anaerobic energy stores. This is the basis of the critical power model, which states that, when the required power exceeds the maximum power of the aerobic system (the so-called critical power), task failure will coincide with the depletion of a fixed anaerobic energy store. The obvious corollary of this is that following task failure, subjects will not be able to continue exercising unless the power output is decreased to below the critical power."

This study tested the above hypothesis. Healthy males (N = 7) performed an incremental test on a cycle ergometer to determine their power at VO2max (P100%), followed by a familiarization trial and four time-to-task failure (TTF) tests at 120% of VO2max (P120%). The first three time-to-task failure tests were identical and used to determine time-to-task failure at P120% (TTF_120). To ensure that TTF_120 represented the maximum performance of Ss at this intensity, only the best time-to-task failure of the three tests was considered. The last time-to-task failure test was similar to the first three, but Ss were then instructed to continue exercising until TTF_120 if possible, and at the moment they reached this time, power output was decreased to P105% and Ss were asked to continue again as long as possible (TTF_105). Capillary blood was sampled before each test, at task failure, and after five minutes of recovery at 60W.

All Ss were able to reach TTF_120 during the final time-to-task failure test. Six Ss were able to continue exercising at P105% for over 25 seconds. There was no significant difference between the blood lactate measures of TTF_120 and TFT_105 measured during rest or at task failure. However, blood lactate was significantly different after five minutes of recovery, although the values were quite low. Heart rate was significantly higher after TTF_105 than after TTF_120. There was no significant VO2 difference after TTF_120 compared to TTF_105.

Implication. After cycling to task failure at P120%, six of seven Ss were able to continue exercising at P105%. Those results cannot be explained by the critical power model, since the model predicts that the anaerobic work capacity must have been depleted at TTF_120, making it impossible to continue exercising at any intensity above critical power, which would have required a further anaerobic contribution. The depletion of a fixed anaerobic energy store does not explain task failure during high-intensity exercise.